The problem with passivating coating compositions is the fact that most paints do not adhere well to the passivated metal surface about 48 hours after passivation. Steel mills engaged in galvanizing sheet metal have traditionally considered metal which had been chrome passivated to be unpaintable after 48 hours unless the passivation coating has been removed by brushing. Thus, they keep two stocks of their product: oiled stock for fabricators who wish to paint the metal soon after receipt and a passivated stock for customers who do not wish to paint the metal. Although it has been discovered recently that many of the passivated galvanized metals that were previously thought unpaintable can actually be painted with a urethane-based primer such as is taught in U.S. Pat. No. 5,001,173, those passivated metals still are not universally paintable.
In the '173 patent, my colleagues and I disclosed an aqueous epoxy resin coating composition containing chromium trioxide. It is an excellent passivant and has had considerable commercial acceptance but this coating, though closer to being universally paintable than other passivating coatings, requires baking to achieve its passivating properties. One-package systems of the '173 composition are stable for the short term but for long term storage the chromium trioxide is supplied in a solution separate from the epoxy resin component. Dibasic acid esters available from DuPont under the general trade designation DBE are taught in the '173 patent as solvents in the resinous portions of the aqueous coating composition and in the total composition to which the aqueous solution of chromium trioxide has been added.
According to European Patent Application No. 0 273 408, the known processes for the production of water-based corrosion resistant coatings for galvanized steel have the disadvantage that a large portion of the hexavalent chromium is not reduced by the organic resin which is present in each of them. Hexavalent chromium may thus be released from the coating on the metal, which limits the utility of the metal. The addition of a water soluble organic compound having a boiling point exceeding 100.degree. C. and at least one CH.sub.2 OH--group and/or at least one .dbd.CHOH--group to a water-based coating agent containing hexavalent chromium, and a film-forming organic polymer prevents the release of hexavalent chromium when in contact with aqueous solutions and prevents the loss of corrosion resistance. The coatings are heated at from 100.degree. to 300.degree. C.
U.S. Pat. No. 3,185,596 teaches corrosion resistance imparting solutions that contain mixtures of hexavalent and trivalent chromium and water soluble or water dispersible polyacrylic acid. Chromic acid is reduced by formaldehyde in aqueous solution. At least 5% and as much as 60% of the hexavalent chromium is reduced. The patent teaches a composition which will yield a coating which has greatly improved corrosion and impact resistance, flexibility, and paint bonding. It exhibits excellent performance under vinyl paints but is not satisfactory under polyester paints.
A modification and extension of the teaching of the '596 patent is taught in U.S. Pat. No. 4,183,772 whereby the coating composition is made to be universally useful under almost all types of paints. A lesser amount of the polyacrylic acid is used and a water dispensable acrylic emulsion polymer is added. The reduction of the hexavalent chromium and the mixing of the polyacrylic acid are accomplished in generally the same way as taught by the '596 patent but mention is made of a reaction of the polyacrylic acid with the chromium after the reduction with formaldehyde. Also, the partial reduction of hexavalent chromium is preferably controlled so that from about 46-50% is reduced although the broader range of 40 to 60% is also taught. Phosphoric acid is said to be an essential ingredient in the composition of the '772 patent rather than optional as in the '596 patent. The ratio of phosphoric acid to chromium compounds is also said in the '772 patent to be critical to the satisfactory performance of the coating under both vinyl and polyester paints.
The reduction of chromic acid by formaldehyde is utilized in U.S. Pat. No. 4,170,671 wherein a water-soluble mixture of polyacrylic acid and a copolymer of acrylic acid and an acrylate is used as an emulsifier in the polymerization of .alpha., .beta.-monoethylenically unsaturated monomers. A mixture of the resulting emulsion and the reduced chromium is said to give a treating liquid for the preparation of a surface so that a subsequently applied coating shows excellent adhesion.
Various types of liquid coating compositions have been applied to metallic substrates and baked thereon in order to protect the substrates against corrosion. Certain of such coatings are applied in conventional metal coil coating processes, and they must be sufficiently adherent and flexible to resist cracking, chipping and peeling. One process which has been utilized for improving the corrosion resistance of various metal substrates generally involves the application of two coatings. The first coating is comprised of a material such as xanthan gum as carrier for the other ingredients of the coating which include a chromium compound such as chromium trioxide and zinc dust. On baking, the xanthan gum contained in the coating becomes water-insoluble. Generally, baked temperatures of at least about 500.degree. F. are required. Over this baked coating is applied a second coating which comprises a zinc rich resin. Such two-step procedures for improving the corrosion-resistance of metal are described in U.S. Pat. No. 4,026,710 (Kennedy) .
U.S. Pat. No. 3,713,904 (Bernath et al.) describes compositions and methods for producing corrosion-resistant and protective coatings on aluminum and aluminum alloys. The coating on the metal substrate comprises an organic resin, an organic solvent, an inorganic hexavalent chromium compound, an oxidizable component, phosphoric acid and strontium chromate. On mixing, the strontium chromate and oxidizable component react to reduce the hexavalent chromium to trivalent chromium. The mixture is applied to the substrate which is then heated at a temperature of from about 600.degree. F. to about 800.degree. F. to achieve a metal temperature of at least about 450.degree. F. which results in the oxidation of a portion of trivalent chromium to hexavalent chromium resulting in a strongly adherent organic resin coating. The organic resins described include epoxy resins. The patentees also describe the application of various topcoats including vinyl topcoats and finish coats comprising strontium chromate-potassium dichromate containing fluorocarbon finish coats.
A washcoat composition suitable for application to tin-plated mild steel is described in U.S. Pat. No. 4,544,686 (Bromley et al.), and the composition consists of an aqueous carrier medium and a binder comprising a thermosetting acrylic polymer, an epoxy resin, an acid catalyst which may be phosphoric acid or chromic acid or an ammonium or amine salt thereof.
According to English language abstracts of Japanese Patent Applications Nos. 59052645A and 59035934A, these publications describe a weldable coated steel sheet which has been obtained by applying a first coating comprising chromium trioxide, about 50% of which has been reduced to the trivalent state, phosphoric acid, polyacrylic acid, and acrylic emulsion in water. The coated metal is then roll-coated with zinc-manganese rich coatings which contain resin such as epoxy resins. The coating then is baked at about 260.degree. C. for one minute.